Moles Moles Moles Moles

  1. Stoichiometry describes the relationships between reactants and products in chemical equations.
  2. A chemical equation can be interpreted in several different ways. The most basic way to think about a balanced chemical equation is to think of it as the simplest recipe for a particular reaction. Take the following equation as an example: CH4 + 3 CuO ---> 3 Cu + 2 H2O + CO. The simplest way of interpreting this is in the following way: 1 molecule of methane reacts with 3 units of copper(II) oxide to form 3 atoms of copper, 2 molecules of water, and 1 molecule of carbon monoxide.
  3. But this is just the basic recipe. We can multiply the recipe many times if we wish, which can lead to new interpretations of the equation:
      CH4 + 3 CuO ---> 3 Cu + 2 H2O + CO
    basic 1 molecule 3 units ---> 3 atoms 2 molecules 1 molecule
    x2 2 molecules 6 units ---> 6 atoms 4 molecules 2 molecules
    x12 12 molecules 36 units ---> 36 atoms 24 molecules 12 molecules
      1 dozen 3 dozen ---> 3 dozen 2 dozen 1 dozen
    x6.02e+23 6.02e+23 1.81e+24 ---> 1.81e+24 1.20e+24 6.02e+23
      1 mole 3 moles ---> 3 moles 2 moles 1 mole
  4. Chemical equations are, in fact, usually interpreted as a representation of the molar ratio of reactants and products in an equation as represented by the last line of the table above.
  5. As long as we keep the ratio of one substance to another we can perform a chemical reaction using any amount of materials that we want. We can use the chemical equation as a guide for adjusting our recipe to match any particular experiment that we might do.
  6. Moles - Moles calculations
    1. Consider the following problem: Given the reaction below, how many moles of aluminum oxide will you produce if you start with 5.40 mol of aluminum?
      4 Al + 3 O2 --->2 Al2O3

    2. First think about what the chemical equation states. The question asks about the relationship between aluminum oxide and aluminum. We can see from the reaction that if 4 moles of Al are used then 2 moles of Al2O3 will be produced. This ratio is fixed, so if we start with 5.40 moles of Al instead, how do we predict the amount of Al2O3 produced?

    3. To do this we must set up a ratio between the two substances asked about in the question.
      5.40       x
      4 Al + 3 O2 ---> 2 Al2O3
      so

    4. By solving for x, you will determine the moles of aluminum oxide produced.
    5. So, the balanced chemical equation can be used to set up a MOLE RATIO between any two substances in the equation.
    6. It is important to remember that you can only set up a

MOLE RATIO!

    1. This is emphasized because you will try to do this with numbers containing other units, like grams. Do not try to set up a ratio using the mass of a substance! This will not work because the coefficients in a chemical equation represent molar quantities (numbers of things), not the mass of things.

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